1) Field of the Invention
The present invention relates to an electrophotographic lithograph printing plate material.
More particularly, the present invention relates to an electrophotographic lithograph printing plate material having an enhanced sensitivity to semiconductor laser rays.
2) Description of the Related Arts
Generally, a conventional electrophotographic lithograph printing plate material has a photosensitive electrophotographic layer wherein electroconductive zinc oxide particles are dispersed as a photoconductive material. This type of lithograph printing plate material (known as a zinc oxide offset master material) is widely employed in the light printing industry, because it is cheap and because the process for making a printing plate from the material is simple and easy.
In a conventional process for producing a lithograph printing plate from the above-mentioned printing plate material, a visible light-irradiation source, for example, a halogen lamp, is used. In this process, the visible light is irradiated to and reflected on an original image or picture and the reflected rays are irradiated to the photosensitive surface of the printing plate material. This method is referred to as a camera system printing plate-making method.
Due to the recent development of various recording machines and the spread of data digitalization, a computer-to-plate type printing plate-making method is now widely used for the electrophotographic material. In this method, laser rays that can be controlled in accordance with computer data are applied to the photosensitive printing plate material surface as a scanning exposure.
Among the laser rays, semiconductor laser rays, which can be generated in a small size device and can be directly modulated, are most useful.
The zinc oxide offset master usable for the semiconductor laser rays is made from a lithograph printing plate material having a photosensitive electrophotographic layer spectrosensitized by a sensitizing dye and having an enhanced sensitivity at a wave-length of 700 to 1000 nm, particularly 780 nm, of the semiconductor laser rays.
Zinc oxide per se exhibits a spectrosensitivity only at a wavelength of about 400 nm. Therefore, to provide a electrophotographic layer having a satisfactory spectrosensitivity at a wavelength of about 780 nm, various sensitizing dye compounds are utilized.
For example, Japanese Unexamined Patent Publication No. 62-220962 discloses sensitizing dye material consisting of cyanine dye compounds having alkylsulfonic acid groups attached, as substituents, to nitrogen atoms located at both terminal portions of the compound molecule. An example of the cyanine dye compounds has the following formula: ##STR3##
The above-mentioned sensitizing dye compounds effectively enhance the spectrosensitivity of the electrophotographic layer when employed together with a chemical sensitizing agent, but this type of electrophotographic lithograph printing material is disadvantageous in that the electrophotographic layer exhibits an undesirably large dark decay in the surface potential thereof.
The procedures for preparing a lithograph printing plate by using laser rays are usually carried out in a continuous system (note, an intermittent system is used by the camera system printing plate-making method) without stopping the printing plate material. Therefore, the effect of the enhanced dark decay is not serious in the continuous printing plate-making method. Nevertheless, when the line speed of the continuous printing plate-making procedure is low, the enhanced dark decay does become a serious problem.
As an example of sensitizing dye compounds exhibiting a small dark decay and a high sensitivity, a compound of the following formula: ##STR4## wherein the nitrogen atoms located at two terminal portions of the dye molecule are substituted by alkyl groups, is known, and this type of sensitizing dye compound is available under the trademark of NK 125, from Nihon Kankoshikiso Kenkyusho.
This type of the sensitizing dye compound, however, has a disadvantage in that it causes the resultant lithograph printing material to exhibit a low heat resistance, and this has an adverse affect on the durability of the resultant lithograph printing material when transported or stored under high temperature conditions. Namely, the commercial value of the lithograph printing material is often significantly reduced by the above-mentioned lower heat-resistance.
Many attempts have been made to provide a sensitizing dye material capable of imparting an industrially satisfactory sensitivity, dark decay resistance and heat resistance to the electrophotographic layer, but these previous attempts did not succeed in obtaining a sensitizing dye material composed of a single dye compound and having all of the above-mentioned properties.